AC-coupled front-end for biopotential measurements

Abstract

Abstract-AC coupling is essential in biopotential measurements. Electrode offset potentials can be several orders of magnitude larger than the amplitudes of the biological signals of interest, thus limiting the admissible gain of a dc-coupled front end to prevent amplifier saturation. A high-gain input stage needs ac input coupling. This can be achieved by series capacitors, but in order to provide a bias path, grounded resistors are usually included, which degrade the common mode rejection ratio (CMRR). This paper proposes a novel balanced input ac-coupling network that provides a bias path without any connection to ground, thus resulting in a high CMRR. The circuit being passive, it does not limit the differential dc input voltage. Furthermore, differential signals are ac coupled, whereas common-mode voltages are dc coupled, thus allowing the closed-loop control of the dc common mode voltage by means of a driven-right-leg circuit. This makes the circuit compatible with common-mode dc shifting strategies intended for single-supply biopotential amplifiers. The proposed circuit allows the implementation of high-gain biopotential amplifiers with a reduced number of parts, thus resulting in low power consumption. An electrocardiogram amplifier built according to the proposed design achieves a CMRR of 123 dB at 50 Hz.

A printed, dry electrode Frank configuration vest for ambulatory vectorcardiographic monitoring

“…The amplifier circuit used in this work is the 222 instrumentation amplifier circuit described by Spinelli et al 223 which has a gain of 1000 [20]. This amplifier circuit was 224 chosen because it has a high common mode rejection ratio 225 (123 dB at 50 Hz) and contains an AC-coupling element that 226 rejects DC drift at the electrodes.…”

A ECG offset cancelling readout circuit using a current mode feedback loop technique

“…1. It consists of AC coupling circuit and three-op amp instrumentation amplifier [1,2]. The ECG detection electrode contains a large DC offset voltage.…”

“…The traditional structure ECG readout circuit uses the three-op amp instrumentation amplifier as the main amplifier, while introducing the integral feedback circuit in the system as a DC offset suppression circuit (DSC) to overcome the DC offset and baseline drift [1,2,3,4]. The main drawback of the three-op amp amplifier is: since the intermediate node contains the input common mode voltage, if there is an offset voltage at the input, the input of the second stage cannot eliminate the offset voltage, the output voltage contains the offset voltage amplified by the second stage amplifier.…”

Towards the standardization of ballistocardiography systems for J-peak timing measurement

“…The bridge output was ac-coupled to a differential amplifier by a fully-differential first-order high-pass filter [17] with cutoff frequency 0.5 Hz, which is high enough to reduce motion artifacts; preliminary tests showed that the frequency components of the J peak were not affected by this filter. The amplifier gain was set to 12,000 and the output was low-pass filtered by a first-order filter with cutoff frequency 100 Hz hence well above the highest frequency components in Figure 2.…”